Micromechanical structure for an acceleration sensor

ABSTRACT

A micromechanical structure for an acceleration sensor, including a seismic mass which is connected to a substrate with the aid of a central connecting element, a defined number of electrodes situated on the substrate, one spring element being situated on each side of the connecting element in relation to a sensing axis.

CROSS REFERENCE

The present application claims the benefit under 35 U.S.C. §119 ofGerman Patent Application No. DE 102015207637.7 filed on Apr. 27, 2015,which is expressly incorporated herein by reference in its entirety.

FIELD

The present invention relates to a micromechanical structure for anacceleration sensor. The present invention also relates to a method formanufacturing a micromechanical structure for an acceleration sensor.

BACKGROUND INFORMATION

Modern sensors for measuring acceleration usually include a siliconmicromechanical structure (“sensor core”) and evaluation electronics.

Acceleration sensors for in-plane movements are available. They includea movable (“seismic”) mass and electrodes. When the mass moves, thedistances between the electrodes change, so that an acceleration may bedetected.

SUMMARY

An object of the present invention is to provide an improvedmicromechanical structure for an acceleration sensor.

This object may be achieved according to a first aspect by amicromechanical structure for an acceleration sensor, including:

-   -   a seismic mass which is connected to a substrate with the aid of        a central connecting element;    -   a defined number of electrodes situated on the substrate;    -   a spring element being situated on both sides of the connecting        element, in relation to a sensing axis.

In this way, the electrodes are situated closer to the sensing axis sothat the arrangement may be less sensitive to a deflection of thesubstrate orthogonally to the sensing axis. Due to the arrangement ofthe spring elements directly at the connection to the substrate, spacefor additional damping structures or springs may be created in theseismic mass.

According to another aspect, the object may be achieved by a method formanufacturing a micromechanical structure for an acceleration sensor,including the steps:

-   -   forming a substrate including electrodes provided thereon;    -   forming a seismic mass;    -   connecting the seismic mass to the substrate with the aid of a        central connecting element; and    -   forming two spring elements on each side of the connecting        element in relation to a sensing axis of the seismic mass.

One advantageous refinement of the micromechanical structure providesthat at least one damping element is situated on the seismic massbetween the two spring elements. In this way, an available space betweenthe two spring elements may advantageously be used for structuraldetails of the micromechanical structure.

Another advantageous refinement of the micromechanical structureprovides that another electrode pair is situated between the two springelements on the substrate. An available space between the two springelements may therefore be utilized advantageously in this way.

Another advantageous refinement of the micromechanical structureprovides that a first electric potential is applicable to firstelectrodes, a second electric potential is applicable to secondelectrodes and a third electric potential is applicable to theconnecting element. In this way a detection structure for amicromechanical acceleration sensor is wired electrically in a suitablemanner.

The present invention including additional features and advantages isdescribed in detail below on the basis of the figures. The same elementsor those having the same function have the same reference numerals. Thefigures are not necessarily drawn true to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a conventional micromechanical structure foran acceleration sensor.

FIG. 2 shows a top view of a conventional micromechanical structure fromFIG. 1 with an indication of electric potentials.

FIG. 3 shows a top view of one specific embodiment of a micromechanicalstructure according to the present invention for an acceleration sensor.

FIG. 4 shows a basic flow chart of one specific embodiment of the methodaccording to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a top view of a conventional micromechanical structure 100for an acceleration sensor having a so-called “semi-central suspension.”Micromechanical structure 100 includes a seismic mass 20 which isfunctionally connected to a substrate 10 situated beneath seismic mass20 with the aid of a centrally situated connecting element 13. Firstelectrodes 11 a, which are wired to one another and applied to a firstelectric potential P1 via connecting elements 11, are situated onsubstrate 10. In addition, second electrodes 12 a are situated onsubstrate 10 which are wired to one another and applied to a secondelectric potential P2 via connecting elements 12. Seismic mass 20 issuspended movably with the aid of two spring elements 21, springelements 21 being each connected to a connecting element 13 viaperforated bar and/or web elements 22 designed with an elongated shape.Mechanical stop elements 14 are provided for limiting a deflection ofseismic mass 20.

Seismic mass 20 therefore has two connecting elements 13 facing downwardtoward substrate 10 so that seismic mass 20 is largely independent ofsubstrate warping. In this way, substrate warping may hardly influenceor distort a sensor signal. The aforementioned substrate warping has thenegative result that electrodes 11 a, 12 a situated on substrate 10 arerotated and/or deflected jointly with substrate 10. There may berelative movements of electrodes 11 a, 12 a relative to one another sothat an acceleration error signal is generated.

One main disadvantage of the conventional structure of FIG. 1 is thatelectrodes 11 a, 12 a are placed on both sides around perforated webelement 22 and therefore have an increased sensitivity to deflections ofsubstrate 10, in particular in the z direction so that the sensitivityincreases with an increase in the distance from the sensing axis whichextends through the two stop elements 14 and the two connecting elements13.

FIG. 2 shows structure 100 from FIG. 1 with an indication of theelectric potentials of electrodes 11 a, 12 a and of connecting element13. All first electrodes 11 a and all second electrodes 12 a arefunctionally electrically wired to one another and in this way have thesame electric potential P1 and P2, respectively. Connecting element 13is applied to ground potential PM. It is apparent that a relativelygreat deal of space is required for the connection of electrodes 11 aand 12 a and their connection to substrate 10. This is due in particularto the presence of perforated web elements 22. It is also apparent thatelectrodes 11 a, 12 a are situated a relatively great distance away fromthe center with connecting elements 13 in relation to the totaldimension of structure 100 and are therefore sensitive to mechanicaldeflections or warping of substrate 10 because warping of substrate 10has greater effects the greater the distance of electrodes 11 a, 12 afrom the sensing axis.

A specific design or arrangement of the two spring elements 21 isproposed so that a “central suspension” for seismic mass 20 isimplemented in this way.

FIG. 3 shows a top view of one specific embodiment of a micromechanicalstructure 100 according to the present invention for a micromechanicalacceleration sensor. It is apparent that, in relation to the sensingaxis of seismic mass 20, a spring element 21 is situated on both sideson connecting element 13. In this way, the conventional perforated webelements 22 are unnecessary, so that additional space is available forstructure 100. Electrodes 11 a, 12 a are connected to substrate 10relatively centrally, so that less dependence on substrate deflectionsor warping for structure 100, in particular in the z direction, is to beexpected. Multiple connecting webs are formed over a transverse area ofseismic mass 20, so that a mechanical robustness of seismic mass 20 maybe increased.

In the space thereby made free between the two spring elements 21, atleast one additional electrode pair 11 a, 12 a may be provided (notshown). Additional structures may optionally also be provided for anoptimized mechanical damping of structure 100 (not shown).

FIG. 4 shows a basic flow chart of one specific embodiment of the methodfor manufacturing a micromechanical structure 100 for an accelerationsensor.

In a step 200, a substrate 10 is formed including electrodes 11 a, 12 aprovided thereon.

In a step 210, a seismic mass 20 is formed.

In a step 220, a connection of seismic mass 20 to substrate 10 isestablished with the aid of a central connecting element 13.

Finally, in a step 230, two spring elements 21 are formed on both sidesof connecting element 13 in relation to a sensing axis of seismic mass20.

In summary, a micromechanical structure for an acceleration sensor isprovided with the present invention, which advantageously provides areduced sensitivity to mechanical warping of the substrate (for example,due to an integration process of the structure into a sensor). Thiseffect is easily achieved due to the arrangement of the two springsdirectly on the connecting element of the seismic mass on the substrate.As a result, an improved sensing characteristic for a micromechanicalacceleration sensor may be achieved thereby.

It is advantageously possible to use the principle described here forother sensor technologies, for example, for piezoresistivemicromechanical acceleration sensors.

Although the present invention has been described on the basis ofconcrete specific embodiments, it is by no means limited thereto. Thoseskilled in the art will thus recognize that manifold modifications arepossible which in the present case have been described only in part ornot at all without departing from the core of the present invention.

What is claimed is:
 1. A micromechanical structure for an accelerationsensor, comprising: a seismic mass connected to a substrate with the aidof a central connecting element; a defined number of electrodes situatedon the substrate; and one spring element situated on each side of theconnecting element in relation to a sensing axis.
 2. The micromechanicalstructure as recited in claim 1, wherein at least one damping element issituated on the seismic mass between the two spring elements.
 3. Themicromechanical structure as recited in claim 1, wherein at least oneadditional electrode pair is situated on the substrate between the twospring elements.
 4. The micromechanical structure as recited in claim 1,wherein a first electric potential is applicable to a first one of theelectrodes, a second electric potential is applicable to a second one ofthe electrodes and a third electric potential is applicable to theconnecting element.
 5. An acceleration sensor including amicromechanical structure, the micromechanical structure comprising: aseismic mass connected to a substrate with the aid of a centralconnecting element; a defined number of electrodes situated on thesubstrate; and one spring element situated on each side of theconnecting element in relation to a sensing axis.
 6. A method formanufacturing a micromechanical structure for an acceleration sensor,comprising: forming a substrate including electrodes, provided thereon;forming a seismic mass; connecting the seismic mass to the substratewith the aid of a central connecting element; and forming two springelements on each side of the connecting element in relation to a sensingaxis of the seismic mass.
 7. The method as recited in claim 6, whereinfirst ones of the electrodes are applied to a first electric potential,second ones of the electrodes being applicable to a second electricpotential and the connecting element being applicable to a thirdelectric potential.
 8. The method as recited in claim 6, wherein atleast one additional damping element is situated on the seismic massbetween the two spring elements.
 9. The method as recited in claim 6,wherein at least two additional electrodes are situated on the substratebetween the two spring elements.
 10. A micromechanical structure,comprising: providing a micromechanical structure including a seismicmass connected to a substrate with the aid of a central connectingelement, a defined number of electrodes situated on the substrate, andone spring element situated on each side of the connecting element inrelation to a sensing axis; and using the micromechanical structure fora micromechanical acceleration sensor.